It can be tricky to figure out if a modem you've purchased on your own is supported by your Internet provider. / Thinkstock

by Rob Pegoraro, Special for USA TODAY

by Rob Pegoraro, Special for USA TODAY

Q: I signed up for Comcast Internet and phone service, bought a modem listed on their approved-devices page, and then couldn't get Comcast to activate it. They said I'd have to prove that I owned the modem, since they only rent out that model. Did I waste my money?

A: This Lompoc, Calif., reader's dilemma pivots on the difference between "we approve this device for use on our network" and "we approve you bringing this device to use on our network."

But attempts to get it activated by the local Comcast franchise failed. She said she was first told the problem was her not subscribing to television as well as Internet and voice - then that this device was only available for rent, and that she'd need to prove she owned it.

When I forwarded her complaint to Comcast PR, the reader reported a quick response from a corporate rep who cited an issue with this model's wireless and offered to refund the amount she'd paid for her own modem.

A few days later, however, the reader noticed that Comcast's listing for the DPC3939 now included a "Special Note," highlighted in red, that "a number" of these devices had been stolen and offered for resale online. The note warned that Comcast could not support a stolen modem - or one "for which you do not have valid legal title."

The reader was not amused. Nor were the handful of other Comcast subscribers to gripe about the same issue on the company's support forums.

Comcast couldn't provide further details on this theft report, but Cisco spokesman Nigel Glennie e-mailed Thursday that "there was a theft of some DPC3939 devices in September 2013."

This model seems to have been the only one flagged as stolen property, but it's not alone in existing in a gray area. Comcast's approved-devices page lists 35 models compatible with the current "DOCSIS 3.0" cable standard. But its "My New Modem" listing of DOCSIS 3.0 hardware eligible for retail purchase - what spokeswoman Jenni Moyer advised customers refer to when shopping - only featured 10 of them.

Glennie said that the DPC3939 was an exclusive for Comcast, but a third-party reseller had heard of no such thing. Direct Modem owner Zach Foster e-mailed that he'd had multiple suppliers offer quotes on that model; he didn't name the firm he'd picked but said his Charlottesville, Va., company had sold "hundreds" of this model through its Amazon storefront since December.

Of those, Foster said about 70% had no issues being switched on by Comcast, 10% required a second try, and 20% wound up getting returned when Comcast franchises balked at activating them.

TIP: DON'T TAKE A REP'S WORD AS GOSPEL

Many tech companies - even those engaged in the communications business - can have trouble communicating clearly to their customers. So it can be tempting to lean on whatever scraps of information you can get from the people you speak to directly on a support line or at a store.

Unfortunately, those are also among the employees less likely to know what's going on, especially at larger companies. In the case of a regionally fractured telecom firm like Comcast - which is both a single massive conglomerate but also a network of local franchises with varying policies - the gap between word-of-mouth and reality can be even wider.

(The same goes for secrecy-obsessed firms like Apple.)

There's an occasional abbreviation at travel sites that's worth remembering if you have somebody tell you something on a phone that seems weird: HUACA, short for "Hang Up And Call Again." If there's really been some major change to the rules at a company, you'll hear about it many more times than once.

But you should also remember that rule if you think you're getting unfairly turned down by a support rep, thank them for their time; hang up, call back; repeat as necessary.